In mass flow rate measuring and control devices, flow resistance elements are used for generating as laminar a flow as possible. These elements produce a low pressure drop which causes a volume flow rate via a bypass duct which is arranged parallel to the principal duct and in which the flow rate sensor is disposed. With a laminar flow, there exists a linear connection between the volume flow rate in the bypass duct and the pressure drop produced by the flow resistance.
In dependence on the desired flow rate per unit of time, flow resistance elements having different opening diameters are required: a small flow resistance is required for high volume flow rates, whereas a high flow resistance is needed for low volume flow rates. According to the prior art, a whole range of flow resistance elements is therefore provided for a mass flow rate measuring and control device, which is involved and expensive.